Generally, CRTs are designed to reproduce the original picture image on a glass screen by receiving the picture image signals from the external and exciting phosphors coated on the screen with electron beams emitted from the electron gun in accordance with the signals.
The electron gun is formed with a triode portion composed of a cathode and first and second grid electrodes, and other focusing and accelerating electrodes. The electrode components are provided with beam-guide holes arranged in line with the cathode.
Thermal electrons emitted from the cathode pass through the first and second electrodes while forming an electron beam. The electron beam is then focused and accelerated through the focusing and accelerating electrodes to thereby land on the screen.
The triode portion of the electron gun acts as a critical factor for a cutoff voltage characteristic and a current density distribution. That is, the electron beam emission efficiency of the electron gun is determined by the geometrical structure of the triode portion and the voltage applied thereto.
In the triode assembly, the hole size and the hole portion thickness of the first grid electrode, the distance between the cathode and the first grid electrode, and the distance between the first and second grid electrodes are largely influential to the electron beam emission efficiency. Particularly when the hole size of the first grid electrode is smaller, the electron beam emission efficiency becomes lowered.
FIG. 3 is a cross sectional view showing main components of an electron gun according to a prior art. As shown in FIG. 3, the electron gun has a triode portion composed of a cathode 24, a first grid electrode 20 and a second grid electrode 22. The electrode components 20 and 22 are provided with beam-guide holes 20a and 22a respectively. The hole 20a of the first grid electrode 20 is usually formed with a diameter smaller than or identical with that of the hole 22a of the second grid electrode 22.
However, in such a state, the emission radius of the electron beam 26 is liable to be changed when the driving voltage applied to the cathode 24 varies. When the electron beams 26 pass through apertures of the shadow mask (not shown) with seriously changed emission radii, they are liable to be interfering with neighboring electron beams and generating a so-called moire phenomenon. The moire phenomenon results in spurious patterns in the reproduced picture images.
In order to overcome the above defects, Japanese Patent Laid Open Publication No. Sho 63-266736 discloses an electron gun with a first grid electrode having a beam-guide hole larger than that of a second grid electrode with increased thickness sufficient for maintaining a good cutoff voltage characteristic.
However, in the above technique, the increased thickness of the beam-guide hole portion of the first grid electrode makes it difficult to prevent the change of the electron beam size pursuant to the change of the cathode driving voltage. That is, the emission radius as well as the current density of the electron beam decreases with the increased thickness of the first grid electrode because the voltage applied to the second grid electrode does not effectively reach the electron emission area of the cathode. Therefore, when the driving voltage of the cathode is changed to display various patterns on the screen, the electron beam size becomes seriously changed due to the weak current density and, as a result, the electron beam lands on the screen with random spot sizes.